So Sorry is a secret boss in Underfell.He is first shown at Hotland in the art club room.
The Underfell version of So Sorry looks the same as the Undertale So Sorry, but wears a black vest.
Unlike the original So Sorry, the Underfell Sorry appears to be selfish, rude, lazy, and stupid as said by Flowey. He appears in the art club room in Hotland. He appears to be aggressive too. He hates mercy, he likes to be alone and hates humans.
He runs into Frisk on purpose and tries to kill them.
In Undertale, the computer clock needs to be set to October 10th to fight So Sorry. However, in Underfell he can be fought any time.
Another difference is that he can be fought in the genocide route.
The fight begins with So Sorry running into Frisk and trying to kill them. His first attack is using his tail. So Sorry also uses paper balls and uses Doodlebog to kill Frisk, but instead heals them, which makes him very annoyed. The fight ends with So Sorry running away from Frisk.
So Sorry doesn't appear in Underfell due to some controversies based around the character, due to the creator of So Sorry having weight gain fetishes.
So Sorry is killed by Frisk, which causes him to melt to his death.
True Pacifist Route
Frisk shows mercy to So Sorry. Since So Sorry hates mercy, he runs away from Frisk, ending the fight.